scd: Add option --all to the SERIALNO command.

* scd/command.c (cmd_serialno): Add option --all.
(open_card_with_request): Implement that option.
* scd/app.c (select_all_additional_applications_internal): New.
(select_additional_application): Add mode to call new function.

This option is currently only useful for Yubikeys and basically
ignored with other cards. Its use is

SERIALNO --all
LEARN --force --multi

which will then print keypairinfo and other stuff for the OpenPGP and
PIV application of a Yubikey. Scute is going to use this to allow
using certificates from OpenPGP and PIV at the same time.

• Signed-off-by: Werner Koch <wk@gnupg.org>